In measuring techniques, measuring of time is of crucial importance, and it is used in numerous engineering applications. A time measuring device which produces a digital result from the timing of analogue signalling events practically carries out an analogue to digital conversion. The known time measuring devices characteristically operate in a way that when analogue signals associated with two events are supplied to the input of a time measuring device, the interrelated time difference between the two, i.e. the delay between the events of the two signals can be determined by a time measuring device. A discrete-time signal set appears on the output of the time measuring device, and this is principally an approximate value of the time difference between the input signals, obtained by quantising. The unit of the analogue to digital conversion performed in measuring the time, i.e. the resolution of the time measuring device, is the shortest time in which the increase or decrease of the input length of time already causes a change in the output value. The currently available time measuring devices are envisaged to determine time differences falling into picoseconds range.
Such a high accuracy time measurement is e.g. required in nuclear physics experiments. Timing chips are made primarily for measuring lifetime and velocity of detected elementary particles, and time measuring devices are used also for examining elementary particles in particle detectors. Further frequent fields of application of time measuring devices are depth measurement, ultrasonic measurement of temperature, density, flow and thickness, and magnetostrictive positioning, but time measuring devices may also be applied in laser rangefinders used in cartography and the construction industry.
Due to circuit integration, a number of time measuring devices have already become marketable today, but most of the devices available in trade can only be used to measure time simultaneously in a few channels only. A channel is generally characterised by two inputs, and the time measuring device measures the time shift of signals appearing on these inputs. These devices are generally implemented on a so-called special-purpose chip, i.e. on an integrated circuit which has been configured and manufactured especially for this task. This increases the production cost and inhibits the configuration of basic parameters.
Most of the earliest solutions to make a time to digital converter (TDC) were based on counting the clock signal cycles that occurred in the period between two events. This construction, however, has significant limitations regarding accuracy, because the length of the clock signal cycle will be the resolution of the measurement.
Other designs were also developed over time and among others scientific papers and patents deal with the structure and operation of high accuracy time-to-digital converter devices. Such devices are disclosed in U.S. Pat. Nos. 5,263,012, 6,181,649 B1 , 6,754,613 B2, 6,850,051 B2, 7,928,888 B1, 8,050,148 B2 and WO 2013/007137 A1. Other devices and systems dealing with measuring of time are disclosed in US 2007/0271538 A1 and US 2011/0282625 A1. The construction disclosed in the latter document has a START and STOP input and is adapted for evaluating time intervals.
In U.S. Pat. Nos. 5,315,566 and 7,317,361 B2 such systems are disclosed, in which several clock signal emitting units are connected to the input of a time measuring device.
In U.S. Pat. No. 7,667,633 B2 a time measuring device adapted for measuring the time difference of two measured signals is disclosed. It is a great disadvantage of the apparatus described in this document and similar known devices that they are only able to measure the time difference between two signals, but they are unable to identify separately the timing of the two measured signals.
The known solutions can be grouped in two categories basically: on the one hand integrated special-purpose circuits, or on the other hand approaches implemented on FPGA and expressly based on a digital circuit are applied in them. The integrated special-purpose circuit approaches cannot be directly adapted to FPGA-based time-to-digital converters, because in many cases they have such integrated circuit elements which are not used in a conventional FPGA. In the known solutions implemented on FPGA, the delays of the wires cannot be well-controlled.
Most known solutions aimed at measuring time and determining the timing of a signal are based on the application of delay components, like for example the time-to-digital converter comprising an inner gate delay array and being disclosed in U.S. Pat. No. 8,219,346 B2. In these solutions, the resolution falls into the magnitude of the delay of the gates. One disadvantage of this solution is that it cannot detect a time increment smaller than the delay of the gates. A further disadvantage of this approach is that the inner gate delay array puts a significant thermal noise burden on the measurement, which may deteriorate the accuracy further.
Such a solution is also disclosed in U.S. Pat. No. 7,884,751 B2, showing the time delays applied between the pluralities of flip-flops, and the shortest such delay is 30 ps, but the delays are characteristically around 100 ps, i.e. the available accuracy of the time measuring device according to the document is rather limited.
In U.S. Pat. No. 8,390,349 B1 such a complex time measuring device or TDC (time-to-digital converter) is disclosed in which delays are applied and the accuracy is enhanced by the combination of various types of time measuring devices. In one part of the time measuring device called ‘stochastic TDC’, the parameter uncertainty resulting from the production of certain elements, e.g. flip-flops, is utilised; i.e. the fact that the various flip-flops cannot be produced absolutely identically from an electronic aspect, e.g. regarding the responses given to signals. In the unit according to the document, the various elements having certain deviation parameters are connected in parallel.
There are tasks where there is a high demand for multi-channel time measuring devices: such a need is e.g. to determine the 3D spatial localisation of events taking place in a particle physics detector with high—even centimetre—accuracy. In other particle physics experiments, it is also necessary to use multi-channel time measuring devices. For such purposes, currently only relatively low accuracy very expensive devices are available. Therefore, a need has emerged for a cost efficiently producible, high accuracy, multi-channel time measuring device adapted for determining timing of a signal, which could even be used for performing measurements physicists were unable to do earlier, and furthermore in multi-channel digital logical analyser devices, where a high resolution is required.